Typically, electrical energy is not produced where it is consumed, so it is necessary to transmit power from generation centres (large power plants) to load centres (cities or industrial facilities). High voltage transmission systems transport electrical energy from its source to the point of consumption.
To ensure reliability of supply, and because of economic and other factors, it is common practice to interconnect transmission systems in different geographic or geopolitical regions. As a result, transmission systems are typically large and complex electrical circuits consisting of hundreds of generation/consumption nodes and thousands of transmission lines. Controlling the flow of power between the nodes in such complex circuits is a challenging problem. It is further complicated by the need to control the voltage at each node to within a small tolerance of a rated value.
Historically, there have been only a few approaches to control transmission systems. Node voltages were controlled by mechanically switched shunt connected capacitor or inductor banks, and the power flow through individual lines was controlled by changing taps on phase shifting transformers and by cancelling line inductance by switching capacitors in series with the line. As the operating life of mechanical switches is inversely proportional to the rate at which switching cycles are performed under load, control of transmission systems was limited to slow sequential reconfigurations designed to reach the desired steady state operating point for a given set of conditions. Dynamic control was not possible, and consequently transients initiated by faults, line and generator outages, or by equipment malfunction, were dealt with by operating the system conservatively and by a practice of over-design. This resulted in considerable underutilization of system capacity.
The advent of power grade thyristors in the early 1970s made it possible to improve upon the classical devices for controlling power systems. Thyristors can be described as one-way switches that begin to conduct when a pulse is sent to their gate. They stop conducting when the current is brought to zero. Thyristors were first used as replacements for mechanical switches, alleviating the problem of reduced operating life due to the number of switching cycles. Applications include thyristor switched capacitors and reactors, and thyristor-based phase angle regulators and tap changers.
Over time, owing to the ability of thyristors to delay the turn-on instant, more sophisticated circuit configurations emerged which allow continuous variation of compensator parameters, including static VAr compensators (SVCs) which allow continuous control of shunt connected reactance, and thyristor controlled series capacitors (TCSCs). Considerable deployment of static VAr compensators began in the mid-1970s and, to date, they are the most commonly used power system compensator. Although their ability to indirectly damp power system transients was recognized early on, traditional practices in system planning and operation dominated the industry throughout the 1970s, and the use of SVCs was limited largely to provide reactive power support.
A characteristic of the power industry is that the demand for power rises steadily, while system upgrades are implemented through large and costly projects. Over the years, energy, environmental, right-of-way, and cost problems have delayed the construction of both generation facilities and new transmission lines, so better utilization of existing power systems has become imperative. In the early 1980s, it was recognized that a change was needed in traditional practices in system planning and operation.
Concurrently, technological advancements in the semiconductor industry led to the production of a power grade gate turn-off thyristor (GTO). The GTO is functionally similar to the thyristor, but can also be turned off by sending a pulse to its gate. The commercial availability of GTOs in the mid-1980s made it possible to construct large voltage-sourced converters (VSCs). In principle, VSCs are capable of generating multiphase alternating voltage of controlled magnitude and phase. On one side they have switching elements (GTOs), and on the other they provide voltage support, typically by way of a DC capacitor.
The application of VSCs in the transmission industry became the subject of considerable research effort in the late 1980s and through the 1990s. The concept of flexible AC transmission system” (FACTS) was conceived, allowing power flow control through AC transmission lines using static converters. Devices used to accomplish this objective (of power flow control) are called FACTS controllers. Examples include the advanced static compensator (STATCOM), the series static synchronous compensator (SSSC), the unified power flow controller (UPFC), and the interline power flow controller (IPFC). A comprehensive review of all compensators, classical and modern, can be found in “Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems”, Narain G. Hingorani, Laszlo Gyugyi, ISBN: 0-7803-3455-8 Wiley-IEEE Press, 1999, the contents of which are hereby incorporated by reference.
Analyzing the numbers of control degrees of freedom and constraints that have to be satisfied offers useful insights into the capabilities of different FACTS controllers. As explained above, VSCs can generate voltage of controllable magnitude and phase. This means that each VSC offers two independent degrees of freedom. When a single converter is interfaced to a transmission line, the two degrees of freedom available for voltage control can be transformed into freedom to control active and reactive power exchanged with the line. While the exchange of reactive power does not impose further limitations, drawing active power in steady state operation requires that the converter be equipped with an energy storage device, which, in most cases, is impractical. Hence, there is a constraint that, in steady state, a single converter must not exchange active power with the line.
STATCOM uses one VSC connected in shunt to the line. With the active power constraint imposed, the control of STATCOM is reduced to one degree of freedom, which is used to control the amount of reactive power exchanged with the line. Accordingly, STATCOM is operated as a functional equivalent of an SVC; it provides faster control than an SVC and improved control range.
An SSSC uses a VSC connected in series with the line. In this case, the active power constraint translates into a requirement that the voltage vector injected by the SSSC must at all times be perpendicular to the current vector. This means that an SSSC is equivalent to a controllable series reactance, i.e., an SSSC can be regarded as the functional equivalent of a TCSC. The SSSC offers faster control, and it is inherently neutral to sub-synchronous resonance.
A UPFC consists of two voltage-sourced converters that share a common DC capacitor. One converter is interfaced in series with the line and the other in shunt. The common DC circuit permits unrestricted exchange of active power between the converters so that active power absorbed from the line by one converter can be supplied to the line by the other. As a result, three degrees of freedom are available, or more precisely, there are four degrees of freedom with one constraint. The UPFC can be used to control the flow of active and reactive power through the line and to control the amount of reactive power supplied to the line at the point of installation.
In its basic configuration, an IPFC consists of two voltage sourced converters interfaced in series with two independent transmission lines. As in the UPFC configuration the converters share a common DC circuit that permits the exchange of active power. By injecting appropriate voltages into the lines, an IPFC can redirect the flow of active power from one line to another, while controlling the amount of reactive power. This concept can be extended without difficulty to N lines.
The shortcoming of all current FACTS controllers is their considerable price. At present, they are well beyond reach of many utilities. Moreover, it is arguable whether improvements in control performance achieved by STATCOM and SSSC justify the replacement of their thyristor-based counterparts. The core functionality provided by an IPFC can be largely accomplished by individual line control using classical compensators.
A UPFC offers control options substantially different from those of classical compensators. Nonetheless, due to the need for two converters, the investment required for UPFC installation discourages widespread deployment. Moreover, given its topology, the UPFC is a self-sufficient device, that is, it can make limited or no use of existing compensators, such as an SVC or switched capacitors.
It is therefore worthwhile to seek alternatives to the compensator that would build upon existing equipment and provide flexible power flow control.